Permanent magnets based on the near equiatomic composition of PtCo have been the magnets of choice in applications where large energy products, corrosion resistance, and fracture toughness are primary design considerations. (See, Newkirk, et al., Transactions AIME, 1950, 188, 1249; and Wohlfarth, Advances in Physics, 1959, 8, 208.) PtCo type magnets were studied and developed in the period from 1950 to 1970. (See Craik, Platinum Metals Review, 1969, 13, 95.) Very little research has been reported in recent years.
In the manufacture of PtCo magnets, rapid solidification processing is employed to produce a refined microstructure. Extended solubilities and metastable phases often result in interesting and useful magnetic properties, as described by Overfelt, et al., IEEE Transactions on Magnetics, 1984, MAG-20, for an Fe.sub.77 Nd.sub.15 B.sub.8 alloy. A reduction in grain size occurs when the alloy melt is rapidly solidified. (See Anderson, et al., Materials Research Soc. Proceedings, 1987, 80, 449; and Livingston, Proc. 8th Intl. Workshop on Rare Earth Magnets, ed. K. J. Strnat, 1985, 423.) Katad and Shimizu found coercivities as high as 1.8 kOe in sputtered thin films of Pt.sub.20 Co.sub.80, and confirmed the correlation between coercivity and grain size: J. Appl. Phys., 1983, 54 (12), 7089.
In recent years, the properties of platinum-cobalt magnetic alloys produced by rapid solidification have been studied at the Vanderbilt University Center for the Space Processing of Engineering Materials, Nashville, Tenn.. Preliminary results of these investigations are contained in an Annual Report of Oct. 1, 1985 to Oct. 31, 1986, identified as the "Engelhard I Annual Report, 1985-1986". As described in this report, samples were prepared with nominal compositions of Pt.sub.50 Co.sub.50, Pt.sub.47.5 Co.sub.47.5 B.sub.5, and Pt.sub.45 Co.sub.45 B.sub.10. The samples melted at the top of a vacuum tube and dropped down the tube for cooling by radiation. Some of the Pt.sub.45 Co.sub.45 B.sub.10. The samples were splat-quenched, that is, the still molten sample impacted a copper plate at the bottom of the vacuum tube to form a splat. The copper plate removed heat from one side of the splat to provide a higher cooling rate than tube cooling alone. After annealing the splat-quenched samples at 600.degree.-650.degree. C., a maximum intrinsic coercivity (H.sub.ci) of around 4.5 kOe was observed after 15 minutes of heat treatment. Coercivity values declined as the heating was continued. By way of comparison, as shown in FIG. 4 on page 38 of the Report, the Pt.sub.50 -Co.sub.50 sample produced by an undercooling procedure gave a maximum coercivity of about 6.7 kOe after heat treatment under the same conditions (viz. 15 minutes at 600.degree.-650.degree. C.).